1. Field of the Invention
The present invention relates to an image coding apparatus and a method thereof, and in particular, in the image coding apparatus and the method thereof for performing coding process by a block unit, relates to a technology for performing coding process corresponding to a plurality of input image sizes.
2. Description of the Related Art
AS a technology for high efficiency coding a moving image, coding methods, such as Motion JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group)-1 and MPEG-2, become commercially practical. Each manufacturer is developing and commercially producing DVD (digital versatile disk) recorders or imaging apparatuses such as digital video cameras, which make the video data recordable by using the coding methods. A user can simply view and listen to the moving image using these apparatuses or personal computers, DVD players, etc.
Also, the method using JPEG as the technology which performs compression coding of the still image is in use. A digital still camera compresses a picked-up image using JPEG, and records the compressed image in a memory card etc.
Furthermore, a coding method of the moving image which can produce further high compression rather than the above-described MPEG-2 etc. is desired. Recently, a coding method called H.264/MPEG-4 part 10 (hereinafter, referred to as “H.264”) has been standardized by ITU-T (International Telecommunication Union—Telecommunication Standardization Sector) and ISO (International Organization for Standardization). Although, H.264 requires many operation amounts of coding and decoding rather than the conventional coding methods, it is known that higher coding efficiency is realized. The arrangement of data processing of H.264 is disclosed in Japanese Patent Laid-Open No. 2004-56827, for example. Generally, the digital still camera can select a pixel number, such as 640×480 pixels, 1600×1200 pixels, etc. and record the picked-up still image with the selected pixel number. In the digital video camera, there are products which can select a video format, such as 480/60i, 720/30p, 1080/60i (i: interlace mode, p: progressive mode) and record the picked-up moving image with the selected video format. An imaging apparatus which performs recording processing of several input image signals with which the pixel numbers are different is disclosed in Japanese Patent Laid-open No. 2002-314870, for example.
Thus, the types and sizes of the images which are input and recorded in the imaging apparatus can consider various cases.
However, for example, in cases where the imaging apparatus adopts the H.264 coding method, the following problems exist.
Since a block size in an integer transform performed at the time of a coding process of H.264 is fixed to a 4×4 pixel block, there is a problem that flexibility is lacking when it corresponds to a variety of input images handled with the imaging apparatus.
Furthermore, regarding the above-described integer transform, in a case where an orthogonal transform is performed by the 4×4 pixel block for the input image is compared with the case where an orthogonal transform is performed by an 8×8 pixel block for the same input image, the orthogonal transform by the 4×4 pixel block has a degradation, for a decoded image of a complicated pattern, rather than the orthogonal transform by the 8×8 pixel block. This is because special frequency resolution of the orthogonal transform by the 4×4 pixel block is lower than the orthogonal transform by the 8×8 pixel block.